The present invention relates to a cordless telephone.
Until now, a cordless telephone composed of a base station connected with a telephone line and a portable telephone communicating with it through wireless. This portable telephone is driven by a battery which can be charged by locating the portable telephone on the base station so that their charging terminals are brought into contact.
Now referring to the drawing, an example of the conventional telephones will be explained below.
FIG. 3 shows the arrangement of a charging part of the conventional cordless telephone. In the charging circuit on the side of a base station as shown in FIG. 3, 41 denote A.C. adapter input terminals to be connected to an A.C. adapter for a D.C. voltage having a predetermined magnitude from an A.C. power source; 42 denotes a constant-voltage circuit; 43 denotes a limiting resistor for limiting a charging current to a predetermined value; and 44 are charging terminals. In the charging circuit on the side of a portable telephone, 45 denotes charging terminals; 46 denotes a diode for preventing a backward current; and 47 denotes a chargeable battery (e.g. Ni-Cd battery) generally called a secondary battery. 48 and 50 denote resistors and 49 denotes a potentiometer-type resistor with a slidable tap; these resistors are connected in series. The voltage across the battery 47 is divided by these resistors and the resultant voltage appears at the tap of the resistor 49. 51 denotes a voltage drop detecting circuit to which the voltage appearing at the tap of the resistor 4 is supplied. 52 denotes control means which is a microcomputer. 54 denotes a light emitting diode (LED) for indicating a voltage drop; 53 is a transistor for driving the LED: and 55 denotes a resistor.
The operation of the cordless telephone thus constituted will be explained below. When a power supply voltage is applied to the A.C. adapter input terminal 41, a constant voltage is output from the constant voltage circuit 42; this constant voltage appears at the charging terminals 44. When the portable telephone is placed on the base station, the charging terminals 45 of the portable telephone are brought into contact with the charging terminals 44 of the base station so that a charging current flows into the battery 47 through the backward current preventing diode 46. Then, the value of the charging current is defined by that of the resistor 43.
In such a construction, the charging current is set for a very small value of about 10% of the battery capacity so that the possible longest life of the battery can be obtained. The charging by this current value is generally called normal charging.
If the voltage appearing at the tap of the resistor 49 becomes lower than the reference voltage set by the voltage drop detecting circuit 51 which the portable telephone is being used, the voltage drop detecting circuit 51 sends a signal to the control means. In response to input of this signal, the control means 52 send a signal to the transistor 53 to blink the LED 54, thus indicating that the voltage across the battery has been reduced.
Meanwhile, with only the normal charging, the charging current during the charging is so small that it takes a relatively long time to fully charge the battery. Therefore, some proposals has been made to implement quick charging. In such proposals, if a battery has been fallen into a voltage reduced state, means for detecting the voltage drop across a battery is used to charge the battery with a larger current larger than the normal current durig a predetermined period, thus completing the charging in a short time.
In the cordless telephone as mentioned above, a battery is provided on the side of a portable telephone whereas a charging circuit is provided on the side of a base station; therefore, charging the battery cannot be implemented unless the portable telephone is placed on the base station. Further, usually, if it is not recognized by the voltage drop detecting circuit that the voltage has been reduced to a value lower than a predetermined value, the above quick charging is not realized. This is because the charging with a large current in an insufficient discharged state may break the battery; only when it is recognized by the voltage drop detecting circuit that the battery has been sufficiently discharged, the quick charging is carried out. Such a construction, however, requires that another voltage drop detecting circuit is provided also on the side of the base station, thus leading to high cost.